(1) Field of the invention
The present invention relates to a system and method for controlling an ignition timing for an internal combustion engine applicable to the internal combustion engine mounted in a vehicle in which a limit value for limiting a difference between a current ignition timing and previous ignition timing values is set according to a acceleration condition of the vehicular engine.
(2) Background of the art
In general, a required ignition timing at which each spark plug ignites and burns air-fuel mixture supplied to a corresponding cylinder is different according to a driving condition of a vehicular engine.
Japanese Patent Application First publications (Unexamined) No. sho 55-139971 published on Nov. 1, 1980 and No. sho 60-162059 published on Aug. 23, 1985 exemplify ignition timing control systems for the vehicular engine.
In the above-identified Japanese Patent Application Publications, calculation equations or table maps are prepared to derive respective basic ignition timings. The equations or tables are mutually different and are selected according to engine driving conditions, such as cranking conditions, no load conditions (idling conditions), load conditions, etc. Therefore, the engine driveability and stability can be improved for each driving condition.
In more details, the ignition timing at the time of no-load condition is set toward a retard angle side according to an engine coolant temperature and engine speed to achieve a stability of a number of engine revolutions per time (engine speed) during engine idling condition. On the contrary, the ignition timing at the time of normal load condition is set toward an advance angle side to achieve a high engine output torque and high engine combustion efficiency. Particularly, as the engine load becomes low (light), an optimum ignition timing is set toward a considerably advance angle side since under such a low load condition a combustion of air-fuel mixture supplied to the engine takes place later than a normal combustion, a large stepwise difference exists between the ignition timing at the time of engine idling and that at the time of a low load condition in a vicinity of the idling condition. Consequently, when the vehicle runs during the light engine load condition in the vicinity of the idling condition, a slight variation of loads (a change in an opening angle of an engine throttle valve) causes an abrupt change in the ignition timing and an abrupt change in an engine output torque, thus generating a vehicle jolting. Therefore, a steering operability of the vehicle can be reduced.
To prevent such a step difference in the ignition timing as described above, one of the ignition timing controlling systems has been proposed in a Japanese Patent Application First Publication No. Sho 54-72326 published on June 9, 1979.
In the Japanese Patent Application First Publication No. sho 54-72326, a limit value is provided in an ignition timing angle so that the ignition timing is changed abruptly at one time such that a width of the change in the ignition timing angle exceeds a predetermined angular displacement (i.e., a difference between the current ignition timing and previous ignition timing is below a predetermined value), in order to prevent the engine stability from damage due to a large change in the engine output torque caused by the abrupt change in the ignition timing. The limit value is set smaller.
However, the previously proposed ignition timing system disclosed in the above-identified Japanese Patent Application Publication No. sho 54-723326 has a problem to be described below.
That is to say, since the limit value is set smaller to prevent the abrupt change in the engine output torque and to improve the engine stability, the ignition timing cannot follow the change in engine load at the time of an abrupt acceleration and then a response characteristic of a rise in the engine output torque becomes reduced. On the contrary, when an importance is placed on the response characteristic to avoid such a trouble, the limit value needs to be set larger. Therefore, a difficulty of setting the fixed limit value exits and the stepwise difference in the ignition timing angles between those at the time of engine idling and light load condition in the vicinity of the engine idling still remains. Consequently, a vehicle shock such as the jolting due to the abrupt change in the engine output torque reduces the vehicle steering operability.